Differences between men and women, from physiological to molecular levels, occur in systems far more diverse than those primarily involved in reproduction. A known paradigm is the sex- specific pattern of gene expression in liver for a broad array of proteins that metabolize steroids and drugs, or function in pregnancy. These distinctions in expression are induced by the sex steroids acting via the pituitary to direct a specific profile of growth hormone secretion. Although studied mostly in rodents, this regulatory axis exists also in humans, where it may impact sex differences in drug sensitivity, incidence of certain liver diseases, or complications in pregnancy. Therefore, it is critical to ascertain mechanisms in such precise tissue-specific and hormonal control. An androgen-independent control in male-specific liver gene expression is revealed by mice carrying rsl alleles (regulator of sex-limitation). Rsl is involved in regulation of the mouse sex- limited protein (Slp) gene, which is normally restricted in expression to adult male liver and kidney. In mice homozygous for rsl, Slp also is expressed in females at puberty. Slp synthesis is also increased in the rsl males, and occurs even in rsl mice lacking androgen receptor, indicating that this regulation is independent of androgen induction. We have shown that the rsl effect is liver-specific and does not act on Slp in kidney, and that it affects the array of male-specific liver genes, including P450s and mouse urinary proteins (MUPs), to cause their expression in females. The rsl variant reveals that wild type Rsl's normal function enforces dimorphic liver gene expression by negative regulation that silences male-specific genes in females. A likely mechanism is via transcriptional repression that acts on Rsl target genes in both sexes, and is overcome in males by puberal hormonal induction. The overall goal of this project is to characterize this novel tissue- and gene-specific negative regulatory pathway and to identify the Rsl gene. We will: I) Clone the Rsl gene based on its chromosomal position, which is already narrowed to a genetic interval of 1.2 cM; II) Define the molecular basis of action by identifying Slp cis-acting elements that are targets of the Rsl pathway; III) Define temporal and spatial interaction between Rsl control and hormonal induction of the target genes. Establishing the basis of the Rsl pathway will enhance our understanding of basic mechanisms of gene expression. Further, Rsl control in mice may yield unique insights into regulation of liver genes in humans.